The use of plastics in various industries has been steadily increasing due to their light weight and continual improvements to their properties. For example, in the automotive industry, polymer-based materials may comprise a significant portion, e.g., at least 15 percent, of a given vehicle's weight. These materials are used in various automotive components, such as, interior and exterior trim and side panels. As the industry seeks to improve fuel economy, more steel and aluminum parts may be targeted for replacement by polymer-based materials.
For example, improvements in the mechanical properties of polymers are desired in order to meet more stringent performance requirements. Such mechanical properties include stiffness, dimensional stability, modulus, heat deflection temperature, barrier properties, and rust and dent resistance. Improved mechanical properties may reduce manufacturing costs by reducing the part thickness and weight of the manufactured part and the manufacturing time thereof.
There are a number of ways to improve the properties of a polymer, including reinforcement with particulate fillers or glass fibers. It is known that polymers reinforced with nanometer-sized platelets or particles of layered silicates or clay can significantly improve the mechanical properties at much lower loading than conventional fillers. (See U.S. Pat. No. 6,469,073 issued to Manke et al. (2002).) This type of composite is termed a “nanocomposite.” More specifically, polymer-silicate nanocomposites are compositions in which nano-sized particles of a layered silicate, e.g., montmorillonite clay, are dispersed into a thermoplastic or a thermoset matrix. The improvement in mechanical properties of nanocomposites is believed to be due to factors such as the increased surface area of the particles.
In its natural state, clay is made up of stacks of individual particles held together by ionic forces. The spacing between the layers is in the order of about 1 nanometer (nm) which is smaller than the radius of gyration of typical polymers. Consequently, there is a large entropic barrier that inhibits the polymer from penetrating this gap and intermixing with the clay. (V. Ginzburg et al., Macromolecules, 200, 33, 1089-1099.) Organically treated clays have been achieved by performing intercalation chemistry to exchange a naturally occurring inorganic cation with a bulky organic cation.
In one process, a number of steps are involved in producing reinforced polymers or polymer nanocomposites. The first step involves a process of conditioning or preparing the clay to make it more compatible with a selected polymer. The conditioning step is performed because the clay is generally hydrophilic and many polymer resins of interest are hydrophobic, thus rendering the two relatively incompatible. A cation exchange is then performed to exchange a naturally occurring inorganic cation with an organic cation. In addition, this process may increase the interlayer spacing between each particle, lessening the attractive forces between them. This allows the clay to be compatible with the polymer for subsequent polymerization or compounding. This preparatory step is known as “cation exchange.” Typically, cation exchange is performed with a batch reactor containing an aqueous solution wherein an organic molecule, usually an alkyl ammonium salt, is dissolved into water along with the clay particles. The reactor is then heated. Once ion exchange takes place, the clay particles precipitate out and are then dried.
Depending on the polymer, a monomer may be further intercalated into the clay galleries. The organically modified clay is then ready for melt compounding to combine the clay with the polymer to make a workable material. Both the polymerization step and the melt compounding step involve known processing conditions in which the particles disperse and exfoliate in the polymer.
Recently, near-critical fluids (see U.S. Pat. No. 5,877,005 to Castor et al.) supercritical fluids have been proposed as candidate media for polymerization processes, polymer purification and fractionation, and as environmentally preferable solvents for coating applications and powder formation. (F. C. Kirby, M. A. McHugh, Chem. Rev., 99, 565-602, (1999).) Moreover, supercritical carbon dioxide has been used as a processing aid in the fabrication of composite materials. (T. C. Caskey, A. S. Zerda, A. J. Lesser, ANTEC, 2003, 2250-2254 (2000).) Generally, any gaseous compound becomes supercritical when compressed to a pressure higher than its critical pressure (Pc) above its critical temperature (Tc). One of the unique characteristics which distinguish supercritical fluids from ordinary liquids and gases is that some properties are tunable simply by changing the pressure and temperature. For example, while maintaining liquid characteristic densities constant, supercritical fluids generally experience faster diffusivity and lower viscosity than a liquid.
Supercritical fluids have been used for delaminating layered silicate materials. (See U.S. Pat. No. 6,469,073 issued to Manke et al. (2002) and U.S. Patent Application Publication No. US 2002/0082331 A1 to Mielewski et al. (2002).) For example, in U.S. Pat. No. 6,469,073, original layered clay structures are swelled or intercaled with supercritical fluid medium to increase the spacing and weakening the bonds between the layers. Upon depressurization, the drastic volume change of the fluid mechanically spreads the layers pushing them apart. The depressurization results in the delaminated structure.
However, there are at least three possible scenarios following the depressurization step. If the layered structure is maintained, the spacing between the layers may increase, may remain the same due to reformation of the weak bonds, or may decrease due to the relaxation and reorganization of organic moieties.
Thus, there is a need to more effectively delaminate particles of silicate, nanoplatelet, nanofiber, or nanotube structures which may be processed with polymers to enhance properties thereof.